Image 1. "Platy" Soil Structure Common with Soil Compaction
- Compaction frequently occurs in soils that are at field capacity.
- Compaction reduces pore space, changes soil structure, and increases soil strength.
- Compaction prevents plant roots from developing normally, inhibits nutrient and water uptake, and causes water management problems.
- There are steps you can take to reduce your risks for soil compaction.
We are anxious to get busy in the fields in the spring. However, performing field operations when soils are too wet can cause problems with soil compaction. These problems are not easy to correct.
The issues caused by compaction have negative impacts on crop yields. Conservative estimates for most cases are 5-10% yield loss, with some situations resulting in over 50% loss.
Soil compaction takes four primary forms, and each has different causes. All forms of soil compaction are negative to the crop. Sound management and patience help most people avoid negative effects. Below we will review the major forms of compaction, how issues can occur, how to identify issues, and how to avoid issues.
Generalities of compaction
Different types of soil compaction have one thing in common – the worst compaction does not occur with heavily saturated soils, but instead with soils that are at field capacity. Field capacity is the point at which the soil will hold water against the force of gravity, although the excess has drained out.
At this point, some of the pore spaces are filled with air instead of water. Enough water is still present to serve as a lubricant between soil particles, allowing them to slide and collapse against each other. As this happens, natural pore spaces also collapse, eliminating oxygen. Crop roots will not grow or function normally in oxygen-deprived soil and beneficial microbial processes cannot occur in the absence of oxygen in the soil.
Issues caused by compaction
With compacted soils, there are two major issues:
- Compaction alters the natural soil structure and increases the physical strength of the soil, preventing crop root systems from fully exploring to take up nutrients and water. Soils that are compacted in layers often will hold surface ponds of water early in the season but can become dry and impenetrable to water later in the season.
- Compaction eliminates air (oxygen) from the soil, so crop roots cannot take up nutrients or water, even when they’re surrounded by it.
'Plow Layer’ compaction is the most common and widely known form of soil compaction. This develops with equipment traffic and use of certain implements, such as the disk. A well-defined layer develops just below the depth of tillage. Look for a distinctive platy or blocky-structured soil layer between six and eight inches deep (see Image 1).
Surface compaction develops when repeated tillage destroys soil structure in the top inches of the soil profile and/or heavy rainfall causes soil particles to settle together into a dense layer. Surface compaction also readily develops with no-till cropping when field traffic occurs before soils adequately dry.
Surface compaction may or may not include soil crusting, and even a thin layer can cause crop problems. Look for soil that appears to be lacking in pore spaces, will not easily crumble in the hands, and has a blocky or platy structure.
Sidewall compaction is another familiar form of compaction. In wet soil, one type of sidewall compaction develops when the opener disk of the planter smears the side walls of the seed slit or furrow which roots have difficulty penetrating. In many cases, shrinkage of the drying soil causes the seed slit to pull open. Look for the distinctive seed slit with smeared walls, and roots that grow in a fan shape as they grow the only direction that they can.
Sidewall compaction can also develop with anhydrous ammonia toolbars and side-dress applicators, again when operations are done under adverse soil conditions. Sidewall compaction may interfere with lateral root growth and may result in a soil fracture that allows nitrogen to be lost to the atmosphere as ammonia gas following application.
Initial diagnosis is as simple as occasionally walking over applied acres immediately following anhydrous ammonia application to see if you can detect an ammonia odor. Use a spade to cut perpendicular sections across the application track to check whether sidewall compaction is occurring.
Deep compaction is the general compaction that develops below the eight to ten-inch depth in the soil. This compaction is usually caused by heavy equipment loads on wet soils or soils at field-capacity. Effects can be detected twenty inches deep in the soil or more. Correction of deep compaction can take many years and often includes deep ripping under dry soil conditions, as well as planting and maintaining alfalfa or other deep-rooted forage or cover crops for several years.
What can I do to minimize compaction issues?
Several practices can help reduce your risks for compaction.
- Avoid working in fields when they are wet.
- Limit traffic by designating traffic lanes in fields.
- Limit your machinery weight. Research indicates that limiting weight of implements to 10 tons/A minimizes compaction below the upper 8-10 inches.
- Reduce tire pressure, consider radial tires, or consider dual tires to help spread out the weight load.
- Improve field drainage.
- Consider conservation tillage systems.
- Consider deep rooted rotational crops or cover crops.
References
USDA Agricultural Research Service ARSU Documents: Soil compaction. https://www.ars.usda.gov/ Accessed 3/3/23.
Raper, R.L. and J. M. Kirby. 2006. Soil Compaction: How to do it, undo it, or avoid doing it. ASAE Distinguished Lecture #30, pp 1-14. AETC, Louisville KY.